This invention relates generally to an ultrasonic oscillator, and particularly to an ultrasonic oscillator suitable for use in various ultrasonic atomizers, for example, fuel injection devices for automobiles.
Ultrasonic oscillators of this character are generally so constructed as to operate safely at a constant speed. They are roughly classified into two types; the load resistor type in which a resistor is connected in series with an ultrasonic vibrator that works as an electroacoustic transducer to drive an ultrasonic vibrator horn provided in an ultrasonic atomizer, and the feedback resistor type in which a resistor is connected in series with a feedback circuit of an amplifier circuit which constitutes the ultrasonic oscillator. FIG. 3 illustrates a typical circuit structure of an ultrasonic oscillator of the load resistor type. The oscillator comprises an amplifier circuit 1; a matching coil 2 and a load resistor R.sub.2 7 connected to the output of the amplifier circuit 1, a balanced bridge circuit having capacitors 3, 4, and 5 and a damped capacity R.sub.1 of an ultrasonic vibrator 6 and connected to said matching coil 2 and said load resistor R.sub.2 7; a feedback circuit which comprises a capacitor 8 and a coil 9 connected in series, and which positively feeds an output signal of the balanced bridge circuit back to the input of the amplifier circuit 1; and an input resistor R.sub.3 10 connected to the input of the amplifier circuit 1.
FIG. 4 illustrates a typical circuit of an ultrasonic oscillator of the feedback resistor type. Comparison of FIG. 4 with FIG. 3 reveals that the circuit of the ultrasonic oscillator of the feedback resistor type is generally the same as that of the load resistor type. The exception is that the feedback resistor type does not have the load resistor R.sub.2 7 employed in the load resistor type; instead, it has a feedback resistor R.sub.4 17 connected in series with the capacitor 8 and coil 9 in the feedback circuit.
The ultrasonic oscillator of the load resistor type has disadvantages. Because the electric current (i.e., vibrator current) of the ultrasonic vibrator 6 flows directly into the load resistor R.sub.2 7 via the capacitor 5, the power loss at the load resistor R.sub.2 7 (i.e., the magnitude of electric power wastefully consumed by the resistor) reduces the efficiency of the oscillator and makes it increasingly difficult to drive the ultrasonic vibrator 6 at a constant speed as the load on the vibrator increases. Such disadvantages common to the ultrasonic oscillators of the load resistor type have been experimentally confirmed by the present inventors.
FIG. 5 shows data measured by the present inventors. In the graph the ordinate represents the ratio of the vibration velocity v to the vibration velocity v.sub.0 under the no-load condition of the vibrator 6. The abscissa represents the ratio of the equivalent series resistance R.sub.1 of the ultrasonic vibrator 6 to the input resistance R.sub.3 10. Curves 11, 12, 13, 14, 15, and 16 represent the vibration velocity characteristics of the ultrasonic vibrator 6 using the ratios of the load resistance R.sub.2 7 to the input resistance R.sub.3 10 as parameters. The curves 11 through 16 represent the vibration velocity characteristics when R.sub.2 /R.sub.3 =0, 0.045, 0.09, 0.225, 0.45, and 0.9, respectively. FIG. 5 indicates that the ultrasonic vibrator 6 begins to work outside the constant-speed operation range as the values of the load resistance R.sub.2 7 and the equivalent series resistance R.sub.1 of the ultrasonic vibrator 6 increase, and that the ultrasonic vibrator 6 operates at a constant speed independently of the equivalent series resistance R.sub.1 only when the load resistance R.sub.2 7 is zero. The data imply that, in addition to the aforementioned defects, the ultrasonic oscillator of the load resistor type has a disadvantage of a low upper limit to the load with which the oscillator can continue the oscillation without coming to a halt (or stall). The oscillatable frequency band width of the amplifier circuit 1 in the steady-state condition, governed by the capacitor 8 and coil 9 in the feedback circuit and by the resistors R.sub.2 7 and R.sub.3 10, is narrower than the resonance frequency band width of the ultrasonic vibrator 6 that varies with changes in the temperature and in the load. A substantial limitation is thus imposed on changing the oscillation frequency according to changes in the resonance frequency of the ultrasonic vibrator 6. A further possibility is that transistors and other semiconductor amplifier elements are damaged by an over-excited input to the amplifier circuit 1.
As described above, the ultrasonic oscillator of the load resistor type has so many disadvantages that it is far from being of practical use. As compared with the load resistor type, the ultrasonic oscillator of the feedback resistor type may be said to be more trouble-free since it dispenses with the load resistor R.sub.2 7 that presents the above-mentioned disadvantages. Nevertheless, the present inventors have found that the feedback resistor type too has problems as described below.
(1) When an element having a resistance of a substantially lower value than the input resistor R.sub.3 10 is used as the feedback resistor R.sub.4 17:
Since the gain .mu..beta. of the circuit system is much greater than 1, the characteristics of the ultrasonic vibrator 6 at the start of oscillation, i.e., during the rise, and the upper limit of the load with which the oscillator continues the oscillation without stall is high. The circuit system further features improved constant-speed operation performance and improved efficiency common to the devices of the feedback resistor type. However, the frequency band width of the amplifier circuit 1 is so narrow that it cannot follow the vibrations in resonance frequency of the ultrasonic vibrator 6 with changes in the temperature and in the load, and semiconductor amplifier elements such as transistors can be broken down due to an over-excited input to the amplifier circuit 1.
(2) When an element having a resistance substantially greater than that of the input resistor R.sub.3 10 is used as the feedback resistor R.sub.4 17:
Contrary to the case (1) above, the amplifier circuit 1 has a broad oscillatable frequency band width, and there is no possibility of the semiconductor amplifier elements such as transistors being broken down by an over-excited input to the amplifier circuit 1. Moreover, the oscillator features the afore-mentioned advantages common to the feedback resistor type. However, since the feedback input voltage to the amplifier circuit 1 is divided by the feedback resistor R.sub.4 17 having a large resistance, the circuit system seldom produces a gain .mu..beta.&gt;&gt;1. The ultrasonic vibrator 6 starts to oscillate only when it is practically under no load, and the oscillator has a low upper limit of load with which it continues the oscillation without coming to a stall.
As stated above, even the ultrasonic oscillator of the feedback resistor type shows variations in its characteristics with the value of the feedback resistance. It behaves contrariwise depending on whether the feedback resistance is small or large.